Modulating device for cathode ray display tube



Sept. 29, 1964 M. H. cRowELl. 3,151,272

MODULATING DEVICE FOR cATHoDE RAY DISPLAY TUBE Filed Dec. 18. 1962 ATTORNEV United States Patent O 3,153,272 MODULATING DER/ECE FR CATHODERAY DHSPLAY TUBE Merton H. Crowell, Morristown, NJ., assigner to BellTelephone Laboratories, Incorporated, New York,

NY., a corporation of New York Filed Dec. 18, 1962, Ser. No. 247,441 9Claims. (Cl. S15-3o) This invention relates to cathode-ray tubes and,more particularly, to apparatus for modulating the electron beams ofcathode-ray display devices.

The copending application of R. J. Beck, Serial Number 219,343, tiledAugust 24, 1962, discloses a unique system for displaying informationfrom a plurality of sources simultaneously on a single cathode-ray tubescreen. A ribbon electron beam is swept periodically across a shieldplate containing a number of elongated slots which are perpendicular tothe plane of the beam and are parallel with its direction of deflection.The beam is thereby divided into a number of segments. A modulating wirein front of each of the slots is connected to a signal source andcontrols the electron flow through the slot. rl`hose electrons that flowthrough the slot impinge upon a phosphor screen, leaving a visibletrace. The intensity of each trace is proportional to the modulatingsignal on the corresponding modulating wire.

One convenient use for this display system is the simultaneous displayof the outputs of a plurality of directional radar antennas, each ofwhich is directed at a different elevation angle. With the horizontaldeflection of the beam synchronized with the transmission of a radarpulse, the horizontal position of the beam at the time of reception ofany reflected pulse is a function of the range of the target. Eachantenna is connected to a different modulating wire in order accordingto elevation angle so that the vertical position of any spot on thescreen indicates elevation angle while its horizontal position indicatestarget range.

The use of higher microwave frequencies, and other refinements, havemade radar antennas so highly directional that it may be feasible to usemore than 100 antennas in one system of the type described above.However, for practical reasons, the single cathode-ray tube of the Becksystem is incapable of displaying that many channels simultaneously. Forexample, if the ribbon beam were ten inches wide, provision for 100modulation channels would require that each beam segment would beseparated by less than one-tenth of an inch, depending on the thicknessof the individual segments. Adjacent beam segments of the Beckmodulation system will overlap if they are that closely spaced. Further,difierential divergence of the various beam segments affects theirintensity and therefore impairs the accuracy of the modulation.

Accordingly, it is an object of this invention to modulate accuratelythe segments of a segmented ribbon electron beam.

It is another object of this invention toincrease the number of segmentsof a normal ribbon electron beam that can be individually modulated.

These and other objects of the invention are attained in an illustrativeembodiment thereof comprising an electron gun for forming and projectinga ribbon electron beam toward a phosphor screen. A pair of deflectionplates periodically deliects the ribbon beam in a direction transverseto its plane. Before striking the screen the beam is divided into aplurality of segments by being cl'rected through a beam formingstructure having a plurality of parallel elongated slots. Each segmentis then individually intensity modulated by a separate signal source.

It is a feature of this invention that a collimation lens be includedbetween the deflection plates and the beam forming structure. The lenshas a converging focal length which is equal to its separation from thecenter of deflection of the deflection plates. Under these conditions,the velocities of all the electrons will be parallel as they approachthe beam forming structure, regardless of the extremes of theirdeflection. This is desirable because it avoids the discrepancies inmodulation that invariably accompany deviations in direction of electronflow.

It is another feature, of this invention that the intensity of each beamsegment be modulated by two rod-shaped modulating electrodes on oppositesides of the segment. The electrodes are maintained at a quiescentdirectcurrent voltage which is barely sufficient to repel all, or most,of the incoming electrons. The modulation signal is directed to bothelectrodes and drives them positively so that the quantity of electronsthat are permitted to pass between the modulating electrodes Varies inproportion to the modulating voltage.

It can be shown that the two modulating electrodes constitute anelectron lens which focuses the beam segment to a crossover point. Aftercrossover, in the absence of other apparatus, the electrons will divergeand overlap on adjacent segments. Accordingly, it is still anotherfeature of this invention that two rod-shaped lens electrodes be locatedon opposite sides of each beam segment between the phosphor screen andthe modulating electrodes. The two `lens electrodes are conductivelyconnected, and like the modulatingelectrodes, constitute an electronlens. A predetermined voltage on each pair of lens electrodes ismaintained such that the focal length of the modulating electrodes plusthe focal length of the lens electrodes is approximately equal to thedistance between the lens electrodes and the modulating electrodes.Under these conditions, the electrons will leave the lens electrodesalong approximately parallel lines, rather than overlapping onto otherbeam segments. j

Each pair of lens electrodes can be coupledrto its corresponding pair ofmodulating electrodes-so that its voltage follows the modulatingvoltage. In this way the above relationship of local lengths can beinsured. Alternatively, the lens electrode can be maintained at aconstant voltage such that the sum of the focal lengths of the lens andmodulating electrodes are equal to the distance between .the lens andmodulating electrodes at the average modulating voltage.

These and other objects and features of my invention will be betterappreciated from a consideration of the following detailed description,taken in conjunction with j the accompanying drawing, in which:

FIG. l is a sectional schematic View of a cathode-ray tube employing theprinciples of the invention;

FIG. 2 is a view taken along lines 2 2 of FIG. l; and

FIG. 3 is an enlarged view of part of the electron beam modulationstructure of -the device of FIG. l.

Referring now to FiGS. l and 2 there is shown a cathode-ray tube llhaving an electron gun l2 for form- 1 ing and projecting a ribbonelectron beam 13 toward a` phosphor screen i4. The beam is maintainedwithin a vacuum by an envelope l5 which is typically made of glass. Theelectron gun comprises, by way of example, a cathode 16, a controlelectrode i7, an accelerating electrode 18 and a focusing lens i9, ofthe type known in the art as an Einzel lens. These elementsare'connected in a known manner to a battery 2li. A pair'of deflectionplates 2l deiiects the beam in a direction transverse to the plane ofthe beam as best seen in FIG-2.

The dellecting plates sweep the beam across modulation apparatuscomprising a beam forming structure 23, a plurality of modulatingelectrodes 24 and a plurality of lens electrodes 25. The beam formingstructure 23 is essentially an electron shield plate containing aplurality of elongated slots. As the beam sweeps the structure Z3, thoseelectrons that flow through the slot deiine a beam segment 27 as shownin FlG. 3. To compensate for differences of velocity direction due todeiiection, a beam collimation lens 2S is included between thedeflection plates and the beam forming structure. As will be explainedlater, the electrons travel in substantially parallel lines normal tothe screen 14 as they leave the collimation lens.

The quantity of electrons that finally reach phosphor screen 14 iscontrolled by modulating electrodes 24. Referring to FlG. 3, electrodes24 are arranged in pairs with the electrodes of each pair beingconductively connected together and situated on opposite sides of thepath of an individual beam segment 27. Each pair of modulatingelectrodes is connected to a separate input signal line. For example,modulating electrode pairs 3?, 3l, and 32 are respectively connected tosignal input lines 34, 35, and 35. As shown in PG. l, all of themodulating electrodes are maintained at a negative direct-currentvoltage with respect to the cathode lli. l-lence, in the absence of aninput signal, they repel the beam segment against beam forming structure23. However, an input signal drives the modulating electrodes to a morepositive voltage which tends to propel electrons toward the phosphorscreen 14.

Referring again to FG. 3, t ere is no signal on input line 36 so watmodulating electrode pair 32 repels its beam segment against beamforming structure 23. A large input signal on input line 3d propels allof the electrons of its beam segment toward screen i4. A smaller signalon input line 35 propels some of the electrons of its segment toward thescreen. Hence, each beam segment 'is eiiiectively intensity modulated byits respective modulating electrode pair. The source of modulationsignals and the deiiection control circuit are not shown; these elementsare part of a system auch as that shown in the aforementioned Beck case,into which the present invention may be incorporated.

Each pair of modulating electrodes acts as an electron lens andtherefore tends to focus its respective beam segment. lt can be shownthat the focal length of this modulating electrode lens is given bywhere M is the focal length of the modulating electrodes underconsideration, VL is the potential gradient at the adjacent lenselectrodes 25, VBE" is the potential gradient at the beam formingstructure 23, and VM is the voltage of the modulating electrodes underconsideration. Assuming that the electrons approach the modulatingelectrodes in parallel lines, they will crossover at the focal point, adistance fM beyond the modulating electrodes, and thereafter diverge. ltshould be pointed out that Equation l presupposes that the modulatinglens is a thin lens. The modulating electrodes define an electricalmodulating plane while the beam forming structure delines a beam formingplane and the lens electrodes define a lens plane. Other structures thatdefine thin electrical planes could alternatively be used.

The purpose of lens electrodes 25 is to eliminate or reduce electrondivergence so that adjacent beam segments do not overlap. To do this,the lens electrodes effectively converge or focus the incoming beamsegments to compensate for their divergence, as illustrated in FIG. 3.It can be shown that if the distance between the modulating electrodesand the lens electrodes is equal to the sum of the focal lengths of themodulating electrodes and the lens electrodes, the lens electrodes willprecisely compensate for diverging effects of the modulating electrodes.rl`his criterion is stated algebraically as where fL is the focal lengthof the lens electrode and d2 is the distance between the modulatingplane and the lens plane. The focal length fL of the lens electrodes canbe given by an equation similar to Equation l:

1 VP -VM' fL- 2VL (3) Where VP' is the potential gradient at thephosphor screen and VL is the voltage of the lens electrode. UsingEquations 3 and 1 the condition of Equation 2 can be expressed as 2V 2Vl M l l L l=d2 VL Vrin VP '-VM To a iirst order Equation 4 can in turnbe expressed in terms of voltages on the various electrodes as whereVB?, VM, and VL are the voltages on the beam forming structure, themodulating electrodes, and the lens electrodes, respectively, d1 is thedistance between the beam forming structure and the modulatingelectrodes, and d3 is the distance between the lens electrodes and thephosphor screen. It is usually desirable that the phosphor screen bealuminized, as is known in the art, to facilitate maintenance of thedesired voltage on it.

Equations 2, 4, and 5 are dilierent representations of the samecondition, Even if this condition is met, hovever, the electrons willleave the lens electrodes in parallel lines, and therefore make awell-defined trace on the phosphor screen, only if they approach themodulating electrodes in parallel lines. The collimation electrodes ZSare therefore important in that they force all electrons to follow pathsparallel with the tube axis regardless of the extremes of theirdeflection; see FG. 2. The condition for this action is that the focallength of the collimation lens must equal the distance between thecollimation lens and the center of deflection of deiiecting plates 2l.

In FIG. 3 the width of the beam segment leaving the lens electrodes isshown as being the same as that between the modulation electrodes. Thismay be desirable because if the beam is originally divided into morethan segments, a large change in width may cause adjacent 'segments tooverlap or make individual segments extremely thin. Unaltered beamsegment width is accomplished by making the focal length of themodulating electrodes equal to the focal length of the correspondinglens electrodes. lf the two focal lengths are not equal but stillfulfill the condition of Equation 2, the electrons leaving the lenselectrodes will be parallel, but their beam segment width will be largeror smaller than that at the modulating electrodes.

As described above, individual beam segments are modulated by varyingvoltages on individual pairs of modulating electrodes. Equation 5 can befulfilled for varying values of modulating voltage VM without changingthe voltages of the phosphor screen VP or the beam forming structure VBFif the voltage of the corresponding lens electrodes VL is changed in apredetermined proportion to changes in VM. The exact proportion, ofcourse, depends on the direct-current voltages on the electrodes and theseparation between them.

One method for varying the lens electrode voltage in accordance with themodulation voltage is shown in FIG. 3 wherein each pair of lenselectrodes is connected to its corresponding pair of modulatingelectrodes through a capacitor. The capacitor permits the lenselectrodes to be maintained at a higher direct-current voltage than themodulating electrodes as is usually advantageous for convenientfuliillment of Equation 5. When a voltage pulse is transmitted to any ofthe modulating electrodes, the voltage of the corresponding lenselectrodes is raised in some predetermined proportion depending on theimpedance of the capacitor. Resistors may also be advantageousiyincluded in the input lines wherever desired to proportion the inputvoltage between the lens and modulating electrodes.

For many applications, it is unnecessary to make any compensatingchanges of the lens electrode voltage. For example, it may be convenientto maintain modulating electrodes 24 at a quiescent direct-currentvoltage of -3 volts, the lens electrodes at +550 volts, and the phosphorscreen at 10,000 volts, each with respect to a cathode voltage of zerovolts. With such a large lens electrode voltage comparatively smallmodulation voltages can intensity modulate the beam segments withoutunduly upsetting the equality of Equation 2 since the sum of the averagefocal lengths of the lens and modulating electrodes will equal thedistance between the modulating plane and the lens plane. The lenselectrodes still sufhciently compensate for the diverging effect of themodulating electrodes to permit a l0 inch Wide electron beam to bedivided into more than 100 segments without overlapping.

lt can be seen that a fair degree of design flexibility is inherent inthe described device. For example, the exact electrode voltages andspacings that are most conveniently employed in a particular tube dependon the requirements that are to be met by the tube. The various electrontrajectories that result from different designs are easily determinableby known principles of electron optics. Although the modulatingelectrodes and lens electrodes are shown as being rod-shaped, otherstructures are readily conceivable for deiining an electrical modulatingpiane and lens plane. Numerous other modifications can be made by thoseskilled in the art without departing from the spirit and scope of theinvention.

What is claimed is:

1. A cathode-ray tube comprising:

means for forming and projecting a ribbon electron beam;

a pair of deecting plates for deflecting the beam;

a collimating lens adjacent the deflecting plates;

an indicating screen;

a beam forming structure between the collimating lens and the screen fordividing therbeam into a plurality of segments;

means for individually intensity modulating each beam segment comprisingtwo parallel modulating rods on opposite sides of the path of eachsegment and means for applying a modulating voltage to the rods;

each pair of modulating rods constituting a iirst electron lens having aiirst focal length;

means for individually focusing each segment cornprising two parallellens rods on opposite sides of each segment between the modulating rodsand the screen;

the lens rods constituting a second electron lens having a second focallength;

the distance between the pair of modulating rods and the pair of lensrods of each beam segment being approximately equal to the sum of therst focal length and the second focal length;

and the iirst and second focal lengths being approximately equal.

2. A cathode-ray device comprising:

means for forming and projecting a beam of electrons which isappreciably wider in one dimension than in the other; v

a pair of deflecting plates for deecting the beam;

means for dividing the beam into a plurality of segments;

means comprising a plurality of rst electron lenses for intensitymodulating each beam segment;

means for indicating the impingement of each beam segment; i

and a second electron lens located between each of the iirst lenses andthe indicating means for focusing each ofthe beam segments;

the distance between the iirst electron lens and the second electronlens being substantially equal to the sum of the focal lengths of thefirst and second electron lenses.

3. A cathode-ray tube comprising:

means for forming and projecting a ribbon electron beam;

a pair of detiecting plates having a -center of deection for deflectingthe beam;

a collimating lens for focusing the beam;

the distance between the center of deflection and the collimating lensbeing substantially equal to the focal length of the collimating lens;

an indicating screen;

a plurality of modulating lenses for selectively intensity modulatingdifferent parts of the beam;

a focusing lens between each of the modulating lenses and the indicatingscreen for focusing each of the said different parts of the beam;

the distance between any modulating lens and its corresponding focusinglens being approximately equal to the sum of their focal lengths.

4. In a cathode-ray tube of the type having an electron gun for formingand projecting a beam of electrons toward an indicating screen, amodulating device for controlling electron impingement comprising: v

means included between the electron gun and the indicating screen fordefining a beam modulating plane;

means included between the beam modulating plane and the indicatingscreen for defining a lens plane;

and means included between the electron gun and the Y modulating lplanefor deru'ng a beam forming plane; the potential gradients along the beampath at its intersections with the planes being substantially given bywhere VM lis the potential gradient at the modulating planefVL is thepotential gradient at the lens plane, VBF' is the potential gradient atthe beam forming plane, VP is the potential gradient at the indicatingscreen, VM is the voltage at the modulating plane, VL is the voltage atthe lens plane, and d2 is the distance between the modulating plane andthe lens plane.

5. In a cathode-ray tube of the type having an electron gunfor formingand projecting an electron beam toward an indicating screen on whichindications are made in response to the impingernent of electrons, amodulating device for controlling electron impingement on the indicatingscreen in response to an input signal from a source comprising:

a pair of modulating electrodes both connected to the signal source andlocated between the electron gun and indicating screen on opposite sidesof a predetermined portion of the electron beam;

a pair of lens electrodes located between the modulat- Y ing electrodesand the indicating screen on opposite sides of the beam portion;

the modulating electrodes constituting a first electron' lens having arst focal length;

the lens electrodes constituting a second electron lens -having a secondfocal length;

the distance between the first lens and the second lens beingapproximately equal to the sum of the first focal length and the secondfocal length.

6. In a cathode-ray tube of the type having an electron` gun for formingand projecting an electron beam toward an indicating screen on whichindications are made in response to the impingement of electrons, amodulatingldevice for controlling electron impingement on the indicatingscreen in response to an input signal from a signal source comprising:

Where VM is the voltage on the modulating electrode, VL is the voltageon the lens electrode, VP is the voltage on the indicating screen, VHFis the voltage on the beam forming electrode, d1 is the distance betweenthe beam forming electrode and the modulating electrode, d2 is thedistance between the modulating electrode and the lens electrode, and d3is the distance between the lens electrode and the indicat- Y ingscreen.

7. A cathode-ray tube comprising:

means for forming and projecting a ribbon electron beam;

means for deecting the beam in a direction transverse to the plane ofthe beam;

an indicating screen;

means for dividing the beam into a plurality of segments;

means for individually intensity modulating each beam segment comprisinga pair of parallel rod-shaped modulating electrodes on opposite sides ofthe path of each beam segment and means for applying an individualmodulating voltage to each of said pairs;

the modulation of each beam segment being characterized by a cross-overof the constituent electrons thereof and a subsequent divergence of thebeam segment;

and means for preventing overlapping of adjacent beam segments due tosuch divergence comprising a pair of rod-shaped lens electrodes locatedbetween each pair of modulating electrodes and the indicating screen.

8. The cathode-ray tube of claim 7 wherein each pair of modulatingelectrodes constitutes a rst electron lens having a rst focal length andeach pair of lens electrodes constitutes a second electron lens having asecond focal length;

the rst focal length being approximately equal to the second focallength and the sum of the first and second focal lengths beingapproximately equal to the distance between the first electron lens andthe second electron lens.

9. A cathode-ray tube comprising:

means for forming and projecting a ribbon electron beam;

a pair of deilecting plates having a center of deflection for deectingthe beam;

an electron collimating lens for focusing the beam;

the distance between the center of deflection and the collimating lensbeing substantially equal to the focal length of the collimating lens;

an indicating screen;

means for dividing the electron beam into a plurality of segmentscomprising an electron shield having a plurality of elongated slotslocated between the collimating lens and the indicating screen;

a plurality of pairs of rod-shaped modulating electrodes for selectivelyintensity modulating different beam segments;

and a pair of lens electrodes between each pair of modulating electrodesand the indicating screen for focusing each of the different segments ofthe beam;

each pair of modulating electrodes being characterized by an averagefocal length and each pair of lens electrodes being characterized by anaverage focal length;

the sum of the average focal length of each of the modulating electrodesand each of the lens electrodes being substantially equal to thedistance between the modulating electrodes and the lens electrodes.

No references cited.

1. A CATHODE-RAY TUBE COMPRISING: MEANS FOR FORMING AND PROJECTING ARIBBON ELECTRON BEAM; A PAIR OF DEFLECTING PLATES FOR DEFLECTING THEBEAM; A COLLIMATING LENS ADJACENT THE DEFLECTING PLATES; AN INDICATINGSCREEN; A BEAM FORMING STRUCTURE BETWEEN THE COLLIMATING LENS AND THESCREEN FOR DIVIDING THE BEAM INTO A PLURALITY OF SEGMENTS; MEANS FORINDIVIDUALLY INTENSITY MODULATING EACH BEAM SEGMENT COMPRISING TWOPARALLEL MODULATING RODS ON OPPOSITE SIDES OF THE PATH OF EACH SEGMENTAND MEANS FOR APPLYING A MODULATING VOLTAGE TO THE RODS; EACH PAIR OFMODULATING RODS CONSTITUTING A FIRST ELECTRON LENS HAVING A FIRST FOCALLENGTH; MEANS FOR INDIVIDUALLY FOCUSING EACH SEGMENT COMPRISING TWOPARALLEL LENS RODS ON OPPOSITE SIDES OF EACH SEGMENT BETWEEN THEMODULATING RODS AND THE SCREEN; THE LENS RODS CONSTITUTING A SECONDELECTRON LENS HAVING A SECOND FOCAL LENGTH; THE DISTANCE BETWEEN THEPAIR OF MODULATING RODS AND THE PAIR OF LENS RODS OF EACH BEAM SEGMENTBEING APPROXIMATELY EQUAL TO THE SUM OF THE FIRST FOCAL LENGTH AND THESEOND FOCAL LENGTH; AND THE FIRST AND SECOND FOCAL LENGTHS BEINGAPPROXIMATELY EQUAL.